Fix: relayd: session destruction does not complete in live mode

Live clients currently hold a reference to their relay_session's trace
chunk through the viewer streams. This causes the destruction of live
session to hang for as long as a client is consuming a session's
contents.

The reason for the hang itself is that the "quiet" rotation performed
as part of the session's destruction can never complete. Indeed, a
rotation completes when the last reference to the trace chunk being
archived is released. Since viewer streams hold references to this
trace chunk, they bound its lifetime to their own lifetime, resulting
in the destruction operation appearing to hang to users of
liblttng-ctl / the lttng client.

Since session rotations are not allowed for live sessions, it is safe
to presume that a session's output folder hierarchy will not change
over its lifetime with the exception of sub-folder additions, such as
in per-PID tracing mode where new traces may be added at any time
within the session's output directory.

The viewer session now copies its corresponding relay_session's trace
chunk to obtain a 'user' mode trace chunk. A user mode trace chunk
will prevent the viewer session from modifying the session output
hierarchy and prevent it from keeping the relay_session's trace chunk
"alive". The viewer stream simply use this trace chunk to perform
their filesystem operations, but they no longer extend the lifetime of
the relayd_session trace chunks.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>